Mathematical model of precipitation of magnesium carbonate with carbon dioxide from the magnesium hydroxide slurry

A mathematical model is proposed for a precipitation process of magnesium carbonate in a heterogeneous stirred tank reactor. The model includes a description of dissolution of Mg(OH)₂, absorption of CO₂ and precipitation of MgCO₃. The Nernst-Planck equation is used in the dissolution model to maintain the mass balance and electroneutrality. The van Krevelen-Hoftijzer expression is introduced to describe the enhancement effect of reaction between dissolved CO₂ and OH^- on the mass transfer rate of dissolution and absorption. In the precipitation model, a simplified population balance equation is solved by a moment method for both dissolving and precipitating particles. Unknown precipitation kinetics parameters for Mg(OH)₂MgCO₃ system are fitted against experimental data and compared with Ca(OH)₂CaCO₃ system. According to the present analysis, the liquid-solid and gas-liquid mass transfer dominate the global rate of precipitation. The precipitation kinetics and pH have strong influences on the concentration of reactants and the yield of precipitation.